The invention relates to polyamide-imide resins, and methods for making the same. More particularly, this invention relates to a method of making a modified polyamide-imide resin which can be applied as a coating material from solutions thereof.
Polyamide-imide resins now enjoy an ever growing usefulness in the electrical industry as insulation coatings, such as magnet wire insulation. These resins have been utilized as both base coats and top coats for conductors such as magnet wire. It is well known that an insulated conductor may be upgraded by superimposing an overcoat of a high molecular weight, linear, strong and thermally stable resin thereon. Such an overcoated, insulated magnet wire is disclosed in U.S. Pat. No. 3,022,200. This patent discloses and claims a magnet wire having a base coat of nonlinear, cross-linked and thermoset polyester insulation and an overcoat of a high molecular weight, linear, strong and thermally stable polyamide-imide resin. The term "polyester resin" as used herein refers to any resin having a plurality of ester groups therein, and includes polyester imide, polyamide ester and polyester-amide-imide resins. Such an overcoated magnet wire has better heat shock, solvent shock and winding properties than the insulated magnet wire without the overcoat.
Although present polyamide-imide and modified polyamide-imide coating materials have provided adequate physical properties when applied to conductors, magnet wires that exhibit improved physical properties at lower cost are highly desirable. Exemplary of these physical properties are the coefficient of friction, moisture resistance, abrasion resistance, thermal stability, flexibility and durability of the coating. Because specific applications of coated conductors such as magnet wire may require coefficients of friction falling within a certain range, the ability to selectively determine the coefficient of friction would be highly desirable. Also, because of specific applications of coated conductors such as magnet wire, high abrasion resistance high moisture resistance and thermal stability such as exemplified by the aromatic polyamide-imide coated magnet wires and a low coefficient of friction as exemplified by Nylon polyamide coated magnet wires at the same time would be highly desirable. Such a material has never before been available as a magnet wire enamel.
In manufacturing polyamide-imide coating materials, a wholly aromatic polyamide-imide is made by a one-step process such as that shown in U.S. Pat. No. 3,790,530. A two-step process may also be utilized such as is shown in U.S. Pat. Nos. 3,884,880 and 3,937,673.
Heretofore in the manufacture of polyamide-imide coating materials the proportion of the aliphatic material (ingredient) has been maintained at amounts of less than 50 mol percent. While this manufacturing limitation has not been impossible to meet, it would be highly desirable to provide an improved polyamide-imide coating and method for making the same that utilizes, respectively, amounts of aliphatic material greater than 50 mol percent such would reduce the cost of the coating material. It would also be highly desirable to provide an improved polyamide-imide coating and method for making the same utilizing amounts of aliphatic material in excess of 50 mol percent that exhibit good properties when compared to prior art polyamide-imide magnet wire enamels.
Heretofore in the manufacture of polyamide-imide coating materials the proportion of the acid reactants and the diisocyanate reactants has been critical. In some processes by which polyamide-imide coating materials are manufactured, an excess of diisocyanate cannot be tolerated inasmuch as the excess diisocyanate undergoes a cross-linking reaction whereupon the polyamide-imide coating material looses flexibility. While the careful measuring of reactants has been achieved in the past to produce highly flexible polyamide-imide reaction products, it would be highly desirable to provide an improved polyamide-imide coating material and method for making the same that can tolerate an excess of diisocyanate reactant without any significant loss in properties.
The term "modified polyamide-imide resin" as used herein refers to that group of resins which contain a plurality of amide groups and a plurality of imide groups therein together with a plurality of groups which are not "building blocks" of a "homopolymer." The modified polyamide-imide resins thus should be contrasted with the "unmodified polyamide-imide resins" which include a plurality of amide groups and imide groups therein and can be referred to as a "homopolymer."
It is well known that the reaction between an acid or an acid anhydride and an isocyanate is a vigorous reaction which is difficult to control. Processes that have been developed heretofore have been difficult to control in commercial sized equipment where agitation, temperature and viscosity control and other control techniques, possible in laboratory glass were not readily applicable. The invention provides a method of making modified polyamide-imide resins in commercial size equipment without the control problems experienced heretofore in prior art processes. The synthesis of aromatic polyamide-imide resins are well known to take place in reaction solutions, the solvents of which consist of solvents such as N-methyl pyrrolidone, dimethyl-acetamide, dimethylformamide, dimethylsulfoxide and the like. Such solvents are those referred to herein as aprotic solvents. The resulting aromatic polyamide-imide resins are characterized by their good thermal stability, toughness, good flexibility and high molecular weight. By this invention, modified polyamide-imide resins are provided having these same properties together with an improved moisture resistance and slipperyness, exhibited by a low coefficient of friction. See for example FIGS. 1 and 2. By the method of the invention, the reaction can be carried out in commercial sized equipment without the control problems heretofore experienced in prior art processes.